Give Moen a call. If they have a lifetime warranty they should send the parts to you free of charge.
I had a Moen bathroom faucet that we paid something like $150 for. One of the handles broke off so I called Moen and they sent the part out free of charge.
After the third time the handle broke I replaced the faucet with a cheap $40 faucet from the local hardware store.
Same thing happened with our Moen kitchen faucet, but it was not the handle that broke. On our kitchen faucet it was the pull out spray hose.
Paid around $175 or so for the Moen kitchen faucet.
After the second time the kitchen faucet broke I replace it with a cheap faucet and never had any more problems.
After them two experiences I decided I would never spend that much again on plumbing fixtures.
Replacement parts may be obtained by calling 1-800-289-6636 (Canada 1-800-465-6130)Moen Warranty Information